The present invention relates to a photovoltaic module, particularly, to a photovoltaic module comprising an improved back cover film so as to improve the environmental resistance.
A photovoltaic module, which is arranged on, for example, a roof, is subjected to a severe outdoor environment and, thus, it is strongly required to improve the environmental resistance of solar cells. Various materials of constituting members of the photovoltaic module have been studied in order to improve the environmental resistance of the solar cells.
The construction of, for example, a thin film photovoltaic module will now be described briefly. Specifically, a thin film photovoltaic module comprises a plurality of solar cells each comprising a transparent electrode layer, a photovoltaic semiconductor layer and a back electrode layer, which are stacked one upon the other on the back surface of a front cover glass. The stacked structure formed on the front cover glass is divided into a plurality of strings to form a plurality of solar cells. Also, these solar cells are connected to each other in series so as to form an integrated photovoltaic module. The back surface of the integrated photovoltaic module is sealed with an encapsulant resin such as an ethylene-vinyl acetate copolymer (EVA). Further, the encapsulant resin are protected by a weather-resistant back cover film laminated thereon.
On the other hand, a crystalline photovoltaic module comprises 20 to 30 solar cells formed by using a single crystalline semiconductor wafer having a small area and arranged on a front cover glass. These solar cells are connected to each other by wires so as to form the crystalline photovoltaic module. The back surface of the crystalline photovoltaic module is sealed with an encapsulant resin such as EVA and is further protected by a back cover film, too.
Conventionally, as a material for forming the back cover film, a fluorocarbon resin film (for example, Tedler manufactured by Du Pont), a laminate of fluorocarbon resin/aluminum (Al)/fluorocarbon resin or a laminate of fluorocarbon resin/polyethylene terephthalate (PET)/fluorocarbon resin has been used.
However, the back cover film consisting of a fluorocarbon resin film alone fails to exhibit a sufficiently high humidity resistance, with the result that the metal members sealed with the encapsulant resin such as an output take-out wire and a back electrode tend to be corroded. On the other hand, the presence of the intermediate Al layer or PET layer enables the laminate of fluorocarbon resin/Al/fluorocarbon resin or the laminate of fluorocarbon resin/PET/fluorocarbon resin to be superior to the film consisting of a fluorocarbon resin alone in the humidity resistance. However, the adhesive strength between the fluorocarbon resin film and the encapsulant resin of EVA is not sufficiently high, with the result that the resin film and the EVA tend to peel from each other over a long period of time. Consequently, the metal members inside the photovoltaic module tend to be corroded because of the moisture intrusion through the peeled portion.
As described above, the conventional photovoltaic module is not satisfactory in the adhesive strength between the encapsulant resin and the back cover film, leaving much room for further improvement of the photovoltaic module in respect of the environmental resistance.
It should also be noted that, if the intermediate Al layer is exposed to the outside in the case of using a laminate of fluorocarbon resin/Al/fluorocarbon resin, the Al layer could be brought into contact with the output take-out wire, with the result that the insulation fails to be maintained.